MCP Secures Multi-Provider OAuth Flows Against Mix-Up Attacks

As organizations integrate multiple identity providers into MCP deployments, OAuth 2.0's lack of issuer verification creates a critical security gap: attackers can intercept authorization codes from one trusted provider and use them at another. RFC 9207's iss parameter solves this by requiring authorization servers to identify themselves and clients to validate the issuer byte-for-byte. This fundamental protocol enhancement directly addresses the OAuth mix-up attack family and becomes essential as MCP's trust model scales beyond single-provider scenarios.